It is possible to change the location where ZBrush will write the QuickSave files. If your system drive is getting full, you can set ZBrush to use a different drive instead. This is done by editing the ZBrushQuickSavePath.txt file. On Windows this file is located in the C:\Users\Public\Public Documents\ZBrushData2020 folder or C:\Users\Public\Documents\ZBrushData2020 folder. On Mac OSX it is located in the Users/Shared/ZBrushData2020 folder.